Have DOMImplementation::document() return reference.

Source/core/css/StyleSheetList.cpp

Index: Source/core/css/StyleSheetList.cpp
diff --git a/Source/core/css/StyleSheetList.cpp b/Source/core/css/StyleSheetList.cpp
index 10f5905ba4e8c1901c991d69e8419a337541515c..068c822f487fc3833aa69f0558101089abc016a0 100644
--- a/Source/core/css/StyleSheetList.cpp
+++ b/Source/core/css/StyleSheetList.cpp
@@ -44,12 +44,12 @@ inline const Vector<RefPtr<StyleSheet> >& StyleSheetList::styleSheets()
 {
     if (!m_treeScope)
         return m_detachedStyleSheets;
-    return document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
+    return document().styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
 }
 
 void StyleSheetList::detachFromDocument()
 {
-    m_detachedStyleSheets = document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
+    m_detachedStyleSheets = document().styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
     m_treeScope = 0;
 }